This invention relates generally to a novel two-piece fastener assembly for mounting a sheet of material to a bolt head or the like. In particular, the invention is directed to such a fastener assembly for use in the mounting of thermal insulation material sheets or panels to axial end faces of a large heat transfer roller, such as a dryer cylinder for drying a paper web or the like.
In the area of paper manufacture or web drying, it is conventional to feed a wet paper web or the like over a series of large cylinders which rotate. Suitable means, e.g. high temperature steam, are provided for heating the interior of the dryer cylinder so that its exterior periphery will dry the web during transport. In this process, a large amount of heat loss has been heretofore experienced, especially at the axial ends of the dryer cylinder, by both radiation and convection. Accordingly, attempts have been made to mount thermal insulation material sheets on end panels of the axial ends of the dryer cylinders to thereby greatly reduce such heat loss. It will be appreciated that minimizing such heat loss also reduces the energy required for operating the drying machinery, and this is desirable in view of increasing energy costs.
Generally speaking, such dryer cylinders include at their axial ends a plurality of protruding bolt heads which serve to affix the axial end panels thereof to the generally cylindrical machine body. Hence, there is little, save these generally metallic end panels and protruding bolt heads, upon which to affix the desired insulating material. The prior art has devised various means and apparatus for accomplishing such affixation. For example, one method involves the use of cup-like permanent magnets which are affixed to the face of the thermal insulation panels with the cup cavities thereof opening outwardly of the panel and sized to the approximate diameter of the bolt heads. The magnets are then spaced at similar circumferential positions to the bolt heads, so as to magnetically affix the thermal insulation material end panel thereto. However, the retention force of such magnetic means is relatively small and there is no assurance that during operation, the thermal insulation material end panels will not inadvertently become disengaged, due to vibration or the like.
One mechanical clamping arrangement is illustrated, for example, in U.S. Pat. No. 4,313,267. Therein a relatively complex cylindrical sleeve arrangement is provided with a suitable locking or set screw to be locked onto the protruding bolt heads. Each such sleeve mounts a further axially protruding externally threaded fastener member which may be fed through the insulating material in a suitable fashion to receive a mating fastener externally thereof. Thereupon, suitable cooperating sleeve members, washers, or the like are utilized to secure the insulating material to this latter protruding fastener member. However, this arrangement is relatively cumbersome and expensive requiring a plurality of machined parts, which require some pre-assembly. Thereafter, some degree of skill and several specialized tools are required in effecting assembly thereof with the dryer end walls.